This invention falls within the general classification of high-chromium abrasion-resistant white irons. In the early 1920's a white iron containing about 25% to 30% chromium and about 2.5% to 3.0% carbon was developed and produced by a number of foundries for castings exposed to severe abrasion, or corrosion plus abrasion, such as occurs in centrifugal pumps handling sand or ground coal slurries. It was found to have much better abrasion resistance than the unalloyed or low-chromium white irons commonly used in abrasive service at that time.
In 1949 a technical paper by R. D. Haworth, (Trans. Amer. Soc. for Metals, 41, (1949), 819-869.) indicated that a 15% chromium white iron, with a martensite-austenite matrix, had better abrasion resistance than the 25% to 30% chromium white iron with a similar matrix. Haworth's wear tests were conducted in a "Rubber Wheel Abrasion Test Machine" (RWAT) which exposed the test specimens to a "low-stress" scratching type of abrasion by wet silica sand.
As a follow-up of Haworth's tests, I investigated a series of 12% to 20% chromium white irons, cast into grinding balls 3-inches to 5-inches diameter, in the "Marked Ball Wear Test" (MBWT) which I had developed and which is described in Metals Technology, Trans. AIME, T.P. 2319, April 1948. The 12 to 20% chromium white irons contained up to about 3% nickel or up to about 4% molybdenum or a combination of about 2% molybdenum plus about 2% nickel, which effectively suppressed the formation of pearlite in the cast balls made from the 12% to 20% chromium white irons.
A large number of MBWT's, on the foregoing series of 12% to 20% chromium white iron balls, were conducted between 1949 and 1955. These tests indicated that for best abrasion resistance, combined with relatively good toughness, the high-chromium white irons should have a microstructure consisting of Cr.sub.7 C.sub.3 -type carbides in a matrix of martensite, or austenite plus martensite. Pearlite in this matrix was undesirable and could be effectively suppressed by additions of up to about 3% nickel, or up to about 3% manganese, or up to about 3% molybdenum, or a combination of about 2% molybdenum plus 1% to 2% nickel. However it was found that the nickel and manganese additions tended to over-stabilize the austenite in the matrix of these irons, which in turn tended to injure their abrasion resistance. On the other hand, the molybdenum addition tended to improve the abrasion resistance of the austenite-martensite matrix in these irons, so molybdenum, in amounts up to about 3%, became the preferred addition for suppression of pearlite in the 12% to 20% chromium white irons. These irons eventually became known in the foundry industry as the 15Cr-3Mo types of white irons and will be so designated here.
It was further found by the wear tests and metallographic studies of the balls used in the MBWT that silicon, which is a common constituent in the 15Cr-3Mo types of white irons, tended to promote the formation of pearlite in the matrix, so its presence in these irons was limited to a preferred range of 0.3% to 0.8%.
During the period from about 1955 to 1965 the use of the 15Cr-3Mo types of white iron in abrasion-resistant castings grew rapidly, so ASTM standards specifications for these 15Cr-3Mo types were prepared and accepted by the ASTM Administrative Committee on Standards, in August 1965. The specification is known as ASTM Designation A532-65T. The composition range covered in this specification was 2.4% to 3.6% carbon, 0.4% to 0.9% manganese, 0.3% to 0.8% silicon, 0.5% (maximum) nickel, 14% to 18% chromium, 2.5% to 3.5% molybdenum, 0.10% (maximum) phosphorus and 0.06% maximum sulfur, balance essentially iron.
In the early 1060's, as the uses for the 15Cr-3Mo white iron expanded into heavy-section castings such as liners in large crushers and grinding mills, it became evident that there was a need to modify the alloy so that it would have greater depth hardening properties, i.e. greater pearlite suppressing power, when slowly cooled during the heat treatment normally required to develop optimum hardness and abrasion resistance in the castings. This was accomplished by the addition of about 1.0% copper or nickel and by raising the preferred chromium content to about 20% (see, for example, U.S. Pat. No. 3,410,682). The change also permitted the molybdenum content to be reduced to about 1.5% to 2.0% which lowered the total alloy cost for this high chromium iron. The modification containing about 1.0% copper is now favored by most producers and users of heavy-section castings and is known and will hereinafter be designated the 20Cr-2Mo-1Cu type of white iron.
Almost concurrently with the development of the 15Cr-3Mo alloy, as related above, A. P. Gagnebin et al., (U.S. Pat. No. 2,662,011) developed a competitive white iron composition with a similar structure of Cr.sub.7 C.sub.3 -type carbides dispersed in an austenite-martensite matrix. The specified composition contained 3.0% to 3.7% carbon, 0.5% to 3.0% silicon, 4.0% to 0.0% nickel and 6.8% to 15% chromium, added in balanced proportions according to the formula: ##EQU1## This alloyed white iron, which is now commonly known as Ni-Hard 4, did not have as good abrasion resistance as the 15Cr-3Mo type or the 20Cr-2Mo-1Cu type of white iron, when tested in the laboratory wear tests or in most conditions of field service. It is generally believed that the somewhat inferior abrasion resistance of Ni-Hard 4 when compared to the 15Cr-3Mo or 20Cr-2Mo-1Cu white irons, is due to the relatively high nickel content of Ni-Hard 4, which tends to over-stabilize the austenite in the matrix of the structure. However, Ni-Hard 4 castings have an advantage for the producer of castings in that they develop their desired pearlite-free structure and are ready for use in their as-cast condition, in contrast to the 15Cr-3Mo or the 20Cr-2Mo-1Cu or the 27Cr types of white iron, which normally require a high temperature heat treatment, followed by an air quench, to develop their optimum abrasion resistance and toughness. For some castings and especially the larger size castings with relatively thick sections or complex configurations, it is difficult to perform the high temperature heat treatment without cracking or fracture of the castings during the heat treatment cycle.
In the early 1970's in cooperation with D. A. Stolk, I invented a nickel-free composition for a high-chromium white iron which, in its as-cast condition, had a matrix structure of austenite which could be partially transformed to martensite and thus hardened by refrigeration. This is described in U.S. Pat. No. 3,941,589. The composition contained about 2.5% to 3.5% carbon, about 2.5% to 3.5% manganese, about 12% to 22% chromium, about 1% to 2% silicon, about 1.5% to 3.0% molybdenum, about 1% to 2% copper, and the balance iron. The relatively high manganese content of 2.5% to 3.5% was used in this composition to suppress the formation of pearlite in the matrix of the cast structure. The abrasion resistance of this alloy in laboratory wear tests was at least equal to, and in some conditions superior to, Ni-Hard 4 in both the as-cast and the refrigeration-hardened conditions. However, this 2.5% to 3.5% manganese composition does not have as good abrasion resistance, when tested in the RWAT, or in gouging abrasion in a jaw crusher, as the alloy of my present invention. Furthermore, the 2.5% to 3.5% manganese composition has been found to have several production and quality control disadvantages, such as insufficient pearlite-suppressing power in heavy-section castings and a tendency for the relatively high manganese content to oxidize and be lost in the slag during the melting operation. As a result, it has not become a commercially popular alloy.